The paradox is that people don't actually implement agile methodologies but rather try to shoehorn them into their waterfall mindset.
I've worked with a team of 10 developers (one of them tech lead) + 1 "PM", using Test-Driven Development. The customer was the PO and was involved in the scrum process. When we had plannings, the TL+PM+PO had already prepared the User Stories for us, so they were easier to guesstimate. The customer/PO took part in reviews of deliverables and provided feedback. Overall it worked nicely.
Most other teams i've worked with were not agile, but fragile, waterfall in disguise. Never works.
Right now i work alone (and one QA) maintaining a legacy project plus handling the CI/CD maintenance and whatever else hits my fan. It's agile when it suits the suits, otherwise they're in charge, i have no say. Just yesterday i was handed a task by the boss of the boss, which will occupy me for a few days. I could go on ranting but i'd hit some character limit.